To meet the recent rapid expansion of the mobile communication market and the demand for multi-functional communication service, new communication systems as typified by IMT2000 have been successively proposed and introduced. There is a tendency for the utility frequency to rise to several giga-hertz and for the frequency bandwidth to increase from 5 MHz to 20 MHz and higher. While the size and power consumption of portable equipment are being reduced, surface acoustic wave (SAW) devices are mainly used as the RF and IF filters. To comply with the new system, the SAW devices are also required to have a higher frequency, broader band, lower loss, and lower cost. Heretofore, SAW devices have cleared the rigorous specification requirements of users through improvements in device design technology and manufacturing technology. Such performance improvement is approaching the limit. This implies that the SAW device will need a substantial technical innovation in the future.
Apart from the development of SAW devices, film bulk acoustic resonators (FBAR) constructed by piezoelectric thin films are capable of basic resonance in a giga-hertz band. However, few outstanding advances have been made on FBAR because it was difficult to prepare piezoelectric thin films of quality and the working precision of piezoelectric thin films and substrates on which they are formed could be increased only a little. However, if filters are constructed using FBAR, miniaturization, low-loss, broadband operation in the giga-hertz band, and monolithic integration with semiconductor integrated circuits would become possible. Therefore, FBAR is of potential worth in the drive to realize very small size portable equipment.
PZT is a lead zirconate titanate (PbZrO.sub.3 --PbTiO.sub.3) solid solution. It is a ferroelectric material having high piezoelectricity. The use of PZT has a possibility of realizing FBAR capable of broad-band operation in a high frequency band. For example, Jpn. J. Appl. Phys., Vol. 36 (1997), pp. 6069-6072, reports FBAR using a polycrystalline PZT thin film formed by the sol-gel method. The PZT thin film described in this report has the composition: Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3.
However, the FBAR described in this article fails to provide resonant characteristics unless a bias voltage is applied to the PZT thin film to induce polarization. The resonant characteristics achieved thereby are insufficient to enable low-loss, broad-band operation in a high frequency band of giga-hertz order. It is thus necessary to improve the electromechanical coupling constant of the PZT thin film.
Studying the epitaxial growth of PZT thin films on silicon substrates, the inventors proposed in JP-A 9-110592 and 10-223476 a method for the epitaxial growth of a PZT thin film on a silicon substrate. The construction of a FBAR device using a PZT thin film is not considered in these patent publications.
Japanese Patent No. 2,568,505 discloses to form PbTiO.sub.3 and La-added PbTiO.sub.3 rather than PZT on MgO single crystal substrates as highly oriented thin films. In this patent, pyroelectric properties of these oriented films are under consideration. Given a high degree of orientation, greater outputs are obtained without a need for poling. The application of such oriented films to FBAR is not referred to in this patent. For the construction of FBAR, the substrate must be worked to a high precision, and a piezoelectric thin film must be formed on a silicon substrate rather than the MgO substrate used in the above patent, in order to enable monolithic integration with semiconductor circuits.
As discussed above, no study has been made on the application of PZT thin film to FBAR. Namely, a FBAR based on a combination of a PZT thin film with a silicon substrate and possessing satisfactory resonant characteristics for broad-band operation is unknown.